FIG. 1 shows a conventional example of a sensor capacitance measurement apparatus to measure a sensor capacitance in the case of electrostatic capacitance changing at various kinds of frequency such as a capacitor microphone. The said sensor capacitance measurement apparatus, as shown in FIG. 1, includes an operational amplifier OP equipped with a feedback resistance Rf, an AC voltage generator OSC that generates AC voltage Vin, a sensor capacitance Cs is connected between an input terminal of the operation amplifier OP and the AC voltage generator OSC via a signal line L.
In the conventional sensor capacitance measurement apparatus shown in FIG. 1, electric current flows through the sensor capacitance Cs by the AC voltage Vin, from the AC voltage generator OSC. Since the input impedance of the operational amplifier OP is ideally infinite and further two input terminals of the operational amplifier OP are in a state of imaginary short, the voltage Vout=−(jωin Cs)·Rf·Vin is outputted from a output terminal of the operational amplifier OP. By executing signal processing to the said output voltage Vout, it is possible to obtain a value corresponding to the sensor capacitance Cs.
The conventional sensor capacitance detection apparatus shown in FIG. 1 uses the resistance Rf as feedback impedance. Assume that Vin=V·sin ωin t and the sensor capacitance Cs changes in response to applied physical quantity by angular frequency ωc with a fixed standard capacitance Cd at the center, namely, Cs=Cd+ΔC·sin ωct and the output voltage Vout can be represented as below.Vout=−Rf[(Cd+ΔC·sin ωct)·ωin·cos ωin t+ΔC·ωc·cos ωct·sin ωin t]V·sin ωin t 
As is apparent from this expression, the output voltage Vout includes a term that is proportional to the angular frequency ωc of the sensor capacitance and has frequency characteristics that depends on changing frequency of the sensor capacitance Cs.
Consequently, it is necessary to set up a processing circuit to cancel the term that is proportional to the said angular frequency ωc at the subsequent stage of the sensor capacitance detection apparatus, and therefore, the size of overall apparatus becomes large.
Then, an apparatus that can obtain the output voltage Vout that does not depend on the angular frequency ωc of the sensor capacitance Cs by substituting the feedback resistance of the operational amplifier OP for a feedback capacitor has been already proposed. FIG. 2 shows a sensor capacitance detection apparatus using the feedback capacitor Cf, and the output voltage Vout of this apparatus can be represented as below.Vout=−(Cd+ΔC·sin ωct)/Cf·V·sin ωint 
As is apparent from this expression, since the output voltage Vout does not have the changing frequency dependency of the sensor capacitance, an additional circuit to cancel the term that is proportional to an angular frequency ωc component is not necessary.
Since the sensor capacitance detection apparatus shown in FIG. 2 uses the feedback capacitor Cf as the feedback impedance of the operational amplifier, the electric current does not come in and out the signal line L that connects the said capacitor Cf and the sensor capacitance Cs. Consequently, since the signal line L is in an electrically floating state, the electric potential becomes unstable, it happens that the circuit output is saturated with the power voltage and there is a problem that the circuit does not operate normally.
The present invention is done to solve the above-described problem of the conventional example and its object is to make it possible to fix the electric potential of the signal line even in the case of using a capacitor as the feedback circuit of the operational amplifier in the sensor capacitance detection circuit.